1. Field of the Invention
The present invention relates generally to optical effect pigments and foils. In particular, the present invention is related to chromatic diffractive pigment flakes and foils which can have a variety of diffractive structures on their surfaces to produce selected optical effects.
2. Background Technology
Various pigments, colorants, and foils have been developed for a wide variety of applications. For example, diffractive pigments have been developed for use in applications such as creating patterned surfaces, and security devices. Diffractive patterns and embossments have wide-ranging practical applications due to their aesthetic and utilitarian visual effects.
One very desirable decorative effect is the iridescent visual effect created by a diffraction grating. This striking visual effect occurs when light is diffracted into its color components by reflection from the diffraction grating. In general, diffractive gratings are essentially repetitive structures made of lines or grooves in a material to form a peak and trough structure. Desired optical effects within the visible spectrum occur when diffraction gratings have regularly spaced grooves at specified depths on a reflective surface.
The color changing properties of diffraction gratings and like structures are well known, particularly when used to form holographic images on continuous foils. One feature of diffractive surfaces as described above is that they perform better with directional illumination in order to be visualized. The continuous and rapid variation in color with viewing angle or illumination angle under a predominant and well collimated light source is due to the angular dispersion of light according to wavelength in each of the orders of the diffracted beams. In contrast, diffuse light sources, such as ordinary room lights or light from an overcast sky, when used to illuminate the diffractive colorant or image, do not reveal much of the visual information contained in the diffractive colorant or image, and what is typically seen is only a colored or non-colored reflection from the embossed surface.
There have been attempts to exploit the optical effects created by such devices by dispersing small fragments of diffractive particles in a transparent vehicle onto irregular printed surfaces. These efforts include a wide variety of diffractive structures that provide dispersion of visible light such that the viewer perceives a different color depending on the viewer orientation with respect to the diffractive surface or the illumination geometry. However, each structure heretofore created has its limitations, such as a glittery appearance that is aesthetically undesirable for many purposes.
For example, Spectratek Technologies Inc. of Los Angeles, Calif. produces a relatively large diffractive flake that produces particles that create varying colors depending on orientation of illumination or view. However, the large size of the flakes also contributes to a distinct sparkle, or “glittery” appearance. Thick flakes also tend to pile up on one another at high angles causing loss of chroma and color variations that act as glitter. Such flakes are described in U.S. Pat. No. 6,242,510, stating that: “[t]he unique ability of the prismatic platelets 18 to reflect light at many angles presents a constantly changing image as the line of site for the viewer is changed. The overall effect is best described as a myriad of small, bright reflections, similar to the radiant sparkle of crystals, crushed glass or even the twinkle of starlight.” (Column 5, lines 56-62).
These particles are described in Spectratek's literature as having a minimum size of 50 by 50 microns. It is because of this relatively large size that they tend to be visible as individual particles. Additionally, because the flake thickness is about 12 microns, a 50 micron particle has an aspect ratio of only about 4:1, and even a relatively large 100 micron particle has an aspect ratio of only about 8:1, thus precluding cooperative orientation with respect to each other and to a substrate. Despite the well recognized need for particulates smaller than 50 microns in many painting and printing methods, neither a reduction in particle size or increase in aspect ratio, i.e., greater than about 8:1, is commercially available. Analysis of these commercial flakes reveals they comprise a metallic foil protected by thick layers of plastic film. The metal layer forms the diffractive structure, which contains linear undulations at a spacing corresponding to about 1,700 to 1,800 lines per mm (ln/mm) with an undulation depth of about 140 nm.
In certain applications the continuous changes in color that can be achieved in a continuous foil form of diffraction grating are more preferred than has been heretofore achieved by flake based pigments. Conventional structures and methods of producing particles with diffractive gratings thereon have rendered such particles unsuitable for achieving the optical features achievable by foil structures. Heretofore, modifications of one structural parameter, while potentially beneficial to optical performance, inevitably have had an adverse impact on another critical characteristic. When the particles are large, disorientation results in a glittery effect. When the particles are small and not well oriented, the multiple colors are no longer distinct but tend to blend in appearance. Thus, even under highly collimated illumination the viewer perceives a washed out color range, rather than bright distinct colors characteristic of a continuous foil.
One attempt to provide more uniform colors, such as is required in color shifting security ink, is described in U.S. Pat. No. 5,912,767 to Lee (hereinafter “Lee”). Lee discloses that particles having a circular arrangement of the diffractive features, with grooves having a frequency of between 1,600 to 2,000 ln/mm (a groove width of 0.4 to 0.6 microns), are necessary to obtain a uniform appearance. In one preferred embodiment Lee discloses that one method of improving the uniformity of the color appearance is modulating the groove spacing with respect to the distance from the center of each particle. However, the circular grating structure is likely to suffer from very low brightness, due to the limited number of effective lines, which represent just a sub-region of very small 20 micron particles, as compared to particles of the same size having a simple linear grating type structure. Further, Lee has no teaching as to particle thickness or groove depth and no quantification of the performance that might provide a motivation to develop an efficient or economic method to produce such complex particles.
U.S. Pat. No. 6,112,388 to Kimoto et al. (hereinafter “Kimoto”) teaches the use of inorganic dielectric layers to protect and stiffen a metallic foil. Kimoto requires a rather thick dielectric layer of 1 micron such that the final particle thickness is between about 2.5 and 3 microns. Since the desirable particle size is 25 to 45 microns, this results in an aspect ratio of between about 10:1 to 22:1. At the lower end of such an aspect ratio there is a greater preponderance for disorientation of the particles with respect to the surface of the coated or painted article, which coupled with the relatively large thickness results in a rougher outer surface. The rougher surface detracts from the appearance and is particularly problematic in many applications, such as automotive paint. Although a thicker top gloss coating may partially mask the roughness, it increases the cost and manufacturing cycle time. Increasing the particle size to improve the aspect ratio would make such particles too large for paint spray applications as well as increase the observable glitter effect. While such particles might be amenable to other painting or printing methods, the particles are highly fragile and friable because the thickness of the metal layer is insufficient to increase the fracture toughness of the inorganic material. Thus, the benefits of a higher aspect ratio may not be achievable in the resultant product.
Embossing metal flakes is one conventional approach to producing diffractive particles. However, the necessity of plastically deforming such flakes in order to obtain a permanent modulation height results in particles that do not have the necessary optical characteristics to produce bright distinct colors. For example, U.S. Pat. No. 6,168,100 to Kato et al. (hereinafter “Kato”) discloses methods of embossing metal flakes with a diffractive relief pattern. FIG. 7 of Kato depicts an actual micrograph of flakes having a groove frequency measured to have about 1,300 ln/mm with a depth of about 800 nm. The flake appears corrugated in that the actual thickness of the metal layer, which is suggested to be within the range of 0.4 to 1 micron, is less than the groove depth. Since the optical performance requires a stable surface microstructure, the embossing process must plastically deform the metal foil, resulting in a significant groove depth in relationship to the foil thickness. While the resulting corrugated structure might be expected to remain flat transverse to the groove direction due to the stiffening effect of the grooves, the flake also appears to have a distinct curvature in the direction of the grooves.
Similarly, U.S. Pat. Nos. 5,549,774 and 5,629,068 to Miekka et al. disclose methods of enhancing the optical effects of colorants by the application of inks, such as metallic flake inks, metallic effect inks, or inks with pigments formed of optical stacks, upon embossed metallic leafing. These patents suggest that such embossed metallic leafing pigments should have a particle size between 10 to 50 microns for compatibility with painting or printing techniques. The frequency of the diffractive features in the case of linear grooves having a sinusoidal shape are disclosed as greater than about 600 ln/mm with a depth that should be less than about 500 nm.
U.S. Pat. Nos. 5,672,410, 5,624,076, 6,068,691, and 5,650,248 to Miekka et al. disclose a process for forming embossed thin bright metal particles with a metallic thickness of 10 to 50 nm. This is accomplished by metallizing an embossed release surface with aluminum. These patents suggest that the frequency of the diffractive features should be between 500 to 1,100 ln/mm and that the same process could be used to make multi-layer thin film optical stacks having the structure corresponding to an embossed carrier film or substrate. Embossment techniques are limited, however, with thin flakes because they can lead to undesirable flake deformation (curvature or departure from flatness) and/or fracture, thereby diminishing the angular resolution of the particulates as well as the overall brightness.
In summary, the conventional technology teaches various ways of making particulates having a diffraction grating type structure that collectively create some color dispersion when reconstituted and applied to the surface of an object. While the conventional diffractive microstructures produce a characteristic angular dispersion of visible light according to wavelength, other aspects of the particle microstructure and micromechanics favor an assembly of such particles having a less desirable glittery or sparkle appearance. This is shown in the final appearance of articles printed or painted with conventional particulates. Such printed or painted articles have an appearance which is apparently limited by the size, thickness and fragility of the particulates.